Increasing demand for transmission capacity, high-speed, and/or long-distance data transmissions among multiple users has pushed the development and deployment of fiber-optic communication systems. Technologies making these feasible include wavelength division multiplexing (WDM), and erbium doped fiber amplification (EDFA). In a WDM system, many different wavelength channels are transmitted simultaneously along the optical fibers or data transmission lines. This dramatically increases the capacity of the transmission system and permits wavelength dependent optical network routing. On the other hand, EDFA amplifies multiple channel optical signals over a wide bandwidth without converting the optical signals into electronic signals and back to optical signals. It also offers several advantages including high gain, low additive noise, and fiber compatibility. However, due to the existence of non-uniform EDFA gain with wavelength, variable channel insertion losses in the components, neighboring channel addition and deletion, unstable laser power, microsecond long gain transients in EDFA cascades, etc., the gain levels of different channels in an optical network are varied. This hinders the proper functioning of optical networks. With the channel power variation, the lower gain channels progressively lose power relative to the higher gain channels, causing a significant power and signal to noise ratio (SNR) differential among the various channels, and limiting the transmission distance and usable amplifier bandwidth.
In addition, as channel powers vary in a dynamic network, several system complications may arise which can potentially cause network failure, such as changes in the input signal powers, drift in component wavelength selectivity, changes in link losses and changes in amplifier gain. Thus, it is advantageous to equalize variation in channel gain level for any wavelength dependent element in the optical transmission path.
Various types of optical dynamic channel equalizer have been proposed or developed in recent years, but most of them are expensive and bulky. FIG. 1 (prior art) shows the general operation of such a dynamic channel equalizer as disclosed by U.S. Pat. No. 6,345,133. In operation, a signal is first applied to a demultiplexer (DMUX) 80 and de-multiplexed into its corresponding channels. Each channel then goes through a dedicated variable optical attenuator (VOA) in an array 81, then all channels are multiplexed by a multiplexer (MUX) 82. After multiplexing, an optical channel monitor (OCM) 83, which includes a tap 831, a DMUX 832 and a detector array 833, and a dynamic equalizer controller 84, is used to monitor the performance of each channel passed through the device, and feed the data to the VOA control unit, which then equalizes the signal of each channel by attenuating each channel to its required level. As can be seen from FIG. 1, such a dynamic channel gain equalizer uses one MUX and two DMUXs, which renders the devices expensive and bulky.
U.S. Pat. No. 5,933,270 discloses a more compact optical equalizer. One embodiment (as illustrated in FIG. 2) comprises a circulator 90, a WDM coupler 91 and a plurality of variable light attenuators 92 that are separately connected to the plurality of ports of the WDM coupler 91. Each attenuator 92 is connected to the input port of an optical coupler (tap) 93 that has one output port connected to a controller 94, and one output port to a reflector 95. Due to the employment of the optical coupler 93, the manufacture of such a device is relatively complicated and expensive.
Therefore, it is desirable to provide an optical channel equalizer that is compact and inexpensive to manufacture. Preferably, such an equalizer requires fewer components than the equalizers of the prior art.